1. Field of the Invention
The present invention relates to an image bearing member and an image forming method, an image forming apparatus, and a process cartridge using the image bearing member.
2. Description of the Background Art
Recent development of data processing systems employing electrophotogtaphy has been of note. In particular, laser printers and digital photocopiers that conduct recording using light by converting data into digital signals have markedly improved in terms of image quality and reliability.
Furthermore, coupled with high speed printing technology, these have been applied to machines such as laser printers and digital photocopiers that are able to produce full-color images.
In light of these developments, image bearing members (photoreceptors, photoconductors) that have a good combination of image quality and durability are particularly preferable.
Given their advantages in terms of the cost, productivity, and pollution, image bearing members made of organic photosensitive materials are widely used for laser printers and digital photocopiers employing electrophotography.
Such organic photoconductors (OPC) are classified into two types, a single-layer structure type and a functionally separated laminate structure type. The first commercialized organic photoconductors were PVK-TNF charge transfer complex type photoconductors, which were of the single-layer structure type.
On the other hand, in 1968, Hayashi and Refensburger independently invented a PVK-a-Se laminate image bearing member. Melz, etc. in 1977 and Schlosser in 1978 made a laminate image bearing member having a photosensitive layer formed of an organic pigment dispersion layer and an organic low molecular weight dispersion polymer layer, with the photosensitive layer made entirely of organic materials.
These laminated image bearing members are also referred to as functionally separated type image bearing members, because the photosensitive layer is separated into a charge generation layer (CGL) that generates charges by absorbing light and a charge transfer layer (CTL) that receives and transfers the charges generated by the CGL to neutralize surface charges.
However, organic image bearing members are vulnerable to abrasion caused by repetitive use in comparison with inorganic image bearing members. Thus, as the surface abrasion of such an organic image bearing member progresses, the image bearing member tends to have problems such that the charge voltages decreases, the photosensitivity deteriorates, background fouling occurs due to scarring on the surface of the image bearing member, image density decreases, and overall image quality deteriorates. Durability has been a large problem for the organic image bearing members.
Moreover, in recent years, the size of the image bearing member has shrunk as printing speeds have increased and image forming apparatuses have become more compact, making good durability an even more pressing problem.
Approaches such as imparting lubricity to the photosensitive layer, curing the photosensitive layer, providing fillers therein, or using a charge transport polymer instead of a charge transport material having a low molecular weight dispersed in the polymer layer are widely known to improve the durability of the image bearing member.
Although successful in preventing abrasion of the photosensitive layer, these approaches cause new problems.
That is, ozone, NOx, and other acidic gases that are produced during repetitive use or depending on surrounding conditions are attached to the surface of the photosensitive layer with repetitive use and depending on use conditions, thereby reducing the resistance of the uppermost surface layer of the image bearing member, resulting in problems such as image flow (image blur). Typically, these materials causing such image flow are scraped away little by little together with the photosensitive layer, so that such problems can be avoided to some degree.
However, new approaches are needed to meet demand for higher resolution and better durability as described above. Heating an image bearing member by a heater is one conceivable approach to reduce the impact of such problems but is not preferable in terms of size reduction of the device and power consumption. Furthermore, adding additives such as anti-oxidants is also effective, but since simple additives have no photoconductivity, addition of additives in large amounts leads to deterioration of electrophotography characteristics such as sensitivity and an increase in the residual voltage.
As described above, a highly durable image bearing member or one which is less vulnerable to abrasion (achieved by process designing around the image bearing member) has inevitable side-effects on image quality, such as image flow and reduction in resolution. Therefore, making an image bearing member having both good durability and with an ability to produce high-quality images has been thought to be difficult to achieve. This is because a high resistance is preferable to reduce the occurrence of image flow whereas a low resistance is preferable to reduce an increase in the residual voltage.
In addition, almost all of the image bearing members currently on the market are of the functionally separated laminate structure type, in which the charge generation layer and the charge transfer layer are laminated on an electroconductive substrate. In addition, the charge transfer material contained in the charge transfer layer is a positive hole transfer material. This arrangement is used in a negative charging electrophotography process.
Furthermore, a charging system using corona discharging is reliable in the electrophotography, and for that reason is employed in most photocopiers and printers. However, as is known, scorotron charging systems are employed because negative corona discharging is not as stable as positive discharging. Therefore, the cost increases. Also, negative corona discharging produces a larger amount of ozone, which causes chemical damage. Therefore, problems easily arise such that ozone produced during charging degrades the binder resin and the charge transfer material contained in the photosensitive layer by oxidization, and ionized compounds produced during charging such as nitrogen oxide ions, sulfur oxide ions, and ammonium ions accumulate on the surface, which has an adverse impact on the image quality. Therefore, most printers and photocopiers employing the negative charging system have ozone filters to prevent the ozone from being discharged to the outside. This also increases the cost. Furthermore, ozone produced in large amounts is environmentally damaging.
To solve such problems, a positive charging image bearing member has been researched and developed. A positive charging image bearing member can reduce the amount of ozone, nitrogen oxides, and the like that is produced. In addition, a positive charging image bearing member used with a currently widely used two-component development agent produces reliably good images with little environment-induced change. This is another advantage of the positive charging image bearing member over the negative.
However, in a positive charging image bearing member having a single layer structure or a reverse laminate structure, the charge generation material, which is extremely susceptible to oxidizing substances such as ozone, nitrogen oxide ions, and the like, is present on or around the surface of the image bearing member and is significantly affected by emission gases from blue heaters, automobiles, etc.
As described above, the negative charging image bearing member is preferable to the positive charging image bearing member for high-speed photocopying processes.
The reason is that almost all of the organic materials having a high charge transferability sufficient for high-speed photocopying processes are currently positive hole transfer materials having only positive hole transfer characteristics. Therefore, the charging property of an image bearing member having a regularly arranged laminate structure in which a charge transfer layer formed of a positive hole transfer material is provided on the surface side is limited to negative charging in principle.
As described above, with regard to the charging polarity, an image bearing member that can be both negatively and positively charged has a wider applicability and is advantageous in terms of cost reduction by reducing the kinds of manufactured image bearing members and in terms of the ability to execute high-speed processing.
Japanese patent no. 2732697 (hereinafter referred to as JP-2732697-B) describes an image bearing member that can be both negatively and positively charged. Although successful in some degree, since the charge transport materials of diphenoquinone derivatives are somewhat slow in terms of charge transferability, the obtained characteristics of the image bearing member are not sufficient for high-speed processing and compact photocopiers and printers. To make matters worse, with repetitive use a machine using this image bearing member produces abnormal images having image flow.
In addition, Japanese patent application publication no. 2000-231204 (hereinafter referred to as JP-2000-231204-A) describes usage of an acid remover consisting of an aromatic compound having a dialkyl amino group for an image bearing member. This compound is preferable in terms of maintaining image quality for an extended period of time. However, this compound has a low charge transport power so that it is difficult to meet the demand for high sensitivity and high-speed processing. Therefore, there is a limit to the addition amount of the compound.
Furthermore, KONICA Technical report vol. 13 (37 page, published in 2000, authored by Itami, et al) describes that stilbene compounds having a dialkyl amino group described in JP-S60-196768-A, JP-2884353-B, etc. are effective to reduce image flow caused by oxidizing gases. However, since this has a dialkyl amino group as a substitution group having a strong mesomeric effect (+M effect) at the resonance portion of the triaryl amine structure serving as the charge transfer site, the overall ionization potential value is extremely small.
Therefore, the charge holding power of the photosensitive layer using the compound as the only positive hole transport material is inferior from the start or deteriorates with repetitive use. This is a fatal defect for commercial viability. If the compound is used in combination with other charge transport materials, since the stiblene compound has a considerably smaller ionization potential than those of the other materials, the stilbene compound serves as a hole trap site for the transported charge, thereby degrading the sensitivity and increasing the residual voltage.
JP-2004-258253-A describes an image bearing member that has improved durability and environment resistance to acidic gases, etc. by containing a stilbene compound and a specific diamine compound without causing deterioration of the sensitivity. However, the image bearing member is not sufficient to achieve high-speed processing or size reduction of the printers, which use an image bearing member having a smaller diameter.
Journal of the Chemical Society, Perkin Transactions 1: Organic and Bio-Organic Chemistry, 21, 2728 (1972), authored by D. W. Jones describes synthesis methods of 1,3-diphenyl-2-phthalimide isoindol with regard to naphtalene tetracarbonic acid diimide-isoindol derivatives and naphthalimide-isoindol derivatives, but there is no mention of applying the synthesized material to an image bearing member.
For these reasons, the present inventors recognize that a need exists for an image bearing member that reliably produces high-quality images while maintaining good durability against repetitive use for an extended period of time and reducing deterioration of image quality caused by a decrease in image density or occurrence of image blur, an image bearing member that can be charged negatively or positively, and an image formation method, an image forming apparatus, and a process cartridge using the image bearing member to improve the speed of printing, achieve overall apparatus size reduction, and reliably produce quality images for an extended period of time with repetitive use.